The invention relates to a wind power plant comprising a tower and a rotor that is mounted on the tower and has at least one rotor blade, and a power conversion unit for generating electrical power from the kinetic energy of the natural movement of air.
The use of wind energy to generate electrical power is increasing in importance. However, a technical difficulty with this kind of environmentally sustainable type of generating electrical power is the unsteadiness of the wind. The problem arising from the use of wind energy, of being able to use wind for power generation that at times only blows weakly, is at the same time foiled by the problem that also very high wind speeds can occur at which the wind power plants have to be shut down to prevent damage to the wind power plants by the aerodynamic forces of the wind. However, a wind power plant is not only damaged by high wind speeds, but also by the turbulence intensity of the wind speed. At the same time, the turbulence intensity is defined as the quotient of the standard deviations of the wind speed variation and the mean value of the wind speed
  I  =            σ              v        m              .  
For areas on the Earth that are not affected by tropical cyclones and exhibit wind speeds as wind gust peak values of not more than 70 m/s, there exist technical solutions that safely protect the plants against damage. In these areas, the turbulence intensity steadily decreases with increasing wind speeds. In a severe hurricane the value of the turbulence intensity is markedly below 10%.
In regions where tropical cyclones occur, the situation is totally different. As experiences with wind farms in southern China over the last years show, wind power plants are not up to cyclone of the classes 4 and 5, i.e. wind speeds of over 60 m/s, if the eye of the cyclone travels through the wind farm. In 2006, 25 out of 28 plants of a wind farm were severely damaged in the province of Zhejiang, and about 10 plants were beyond use. Here the rotor blades broke off, nacelles fell from their tower, towers snapped, tower flanges were torn apart, and even foundations were pulled out of the ground. The maximum wind speeds were measured to be 86 m/s. Most wind power plants are only designed for a maximum wind speed Vw of 60 m/s with a safety margin of 1.35 for the loads.
The last years have shown that the number of cyclones is rising strongly and therefore also the danger of greater damage in wind farms that are at risk. Due to the strongly rising spread of wind power use worldwide, wind power plants are also increasingly erected in regions with a risk of cyclones (e.g. India, China, Taiwan, Japan etc.).
What is particular about cyclones is, that not only does the wind speed rise strongly in the wall of the eye of the cyclone but at the same time the turbulence intensity increases to an extreme extent. The wind speeds can reach 90-100 m/s with at the same time a turbulence value intensity of 60%. A rotor blade that is hit by such an air movement transverse to the longitudinal axis of the blade has no chance of surviving this without any damage. The aerodynamic excitation by such a strongly turbulent current leads to extreme oscillations in the blade structure in the edgewise and flapwise directions that are very likely to lead to a total destruction.
Dimensioning the structure with respect to such high loads would lead to very high blade weights and also increase the costs of the whole plant significantly and thus degrade the cost-effectiveness markedly.
A particular problem also occurs in the event of offshore plants that have been erected in regions with a risk of cyclones. Since they are erected in the water off the coast they would be hit by the cyclones at a point when their energy has not yet diminished and the wind speeds are still higher. For a larger number of offshore wind farms in these regions there is a high likelihood that there will be considerable damage each year. This is not acceptable since the operating and repair costs would rise dramatically and as a result insurance companies would not insure such projects and banks would not finance such an undertaking.
In the meantime, different measures have been suggested that transfer in the event of very high wind loads the rotor blades and/or the rotor resp. the nacelle into a position in which these components of the wind power plant present a small area of attack for the wind and thus assume a position with low wind resistance. For example DE 100 58 076 A1 shows such a plant that brings the rotor blades, when a strong wind comes up, vertically to the direction of the wind into the feathering position and rotates the rotor from the upwind into the downwind position when the wind force increases. Thus the plant can track the wind and therefore presents the smallest wind resistance for the entire plant. When considering the individual rotor blade, then despite this it can be flown against transverse to the longitudinal axis as a function of the rotor position. Due to the strong turbulence in a cyclone there will nonetheless be a strong excitation of the structure. Destruction continues to be very likely.
A further invention known from EP 0 709 571 A2 transfers the nacelle with a two-blade rotor into a position relative to the wind direction such that the one blade is flown against from the blade tip. The approach to reduce the loads decidedly is given, however in practice the system would not function since the system is unstable. A deviation of the direction of the wind relative to the longitudinal axis of the blade would create additional moments that the active wind-direction tracking unit must correct in a highly dynamic manner. If this cannot be achieved quickly enough, the moments that want to rotate the rotor at right angles to the direction of the wind in the downwind direction of the tower will become ever greater and the system turns unstable. The conditions of a cyclone will also not be survived by such a configuration.